1. Field of the Invention
The present invention relates to imaging apparatus capable of changing frame rate upon imaging, and a method and a program for controlling the same.
2. Related Art
Conventionally, there has been imaging apparatus capable of changing frame rate upon imaging. In such imaging apparatus, an exposure time per frame is limited to within an inter-frame spacing determined by the frame rate. In addition, in the case in which recorded moving images are reproduced to be watched, image continuity between frames weakens when the ratio of the exposure time to the inter-frame spacing is small. This leads to lose smooth image reproduction. Therefore, while moving images are recorded, automatic exposure (AE) control to adjust the exposure time so that the ratio of the exposure time to the inter-frame spacing increases as much as possible is often conducted. The frame rate increases upon imaging while such automatic exposure (AE) control is being conducted, and then the exposure time per frame shortens. On the other hand, in typical automatic exposure control, exposure control values such as an aperture value, a shutter speed, and an amplification factor upon imaging in the subsequent frame is determined in accordance with the brightness of the image imaged in the previous frame. Thus, the change cannot be followed up even if the exposure time changes with a change of the frame rate as described above, whereby the image immediately after a change of the frame rate often significantly darkens.
As an art for solving the above-mentioned problem, for example, the Japanese Published Unexamined Patent Application Publication No. 2007-195038 discloses that, a correction amount for offsetting the changed portion (decrease) of the exposure time is calculated when the exposure time shortens, owing to the increasing frame rate of the subsequent frame. The above-mentioned patent application further discloses the art that the exposure control values (i.e., controlled variables) such as the aperture value and the amplification factor of the output signal once determined are preliminarily corrected by adding the above-mentioned correction amount in accordance with the level of an output signal of an image sensor, and then an exposure is controlled by using the corrected aperture value and the amplification factor of the output signal when the subsequent frame is imaged. According to the above-mentioned art, a change of brightness of the image imaged when the frame rate changes to be higher upon imaging can be prevented.
However, in the above-mentioned art, it is not necessarily the case that the corrected exposure control values and the combination thereof are appropriate for subsequent imaging because the exposure control values such as the aperture value, the shutter speed, and the amplification factor of the output signal, which are to be set immediately after a change of the frame rate, are corrected uniformly by the correction amount for offsetting the changed portion of the exposure time in response to a change of the frame rate.
That is, in exposure control, for example, when control conditions of maintaining a low amplification factor of the output signal to prevent an image quality from deteriorating and of reducing the frequency of changes of the aperture value is reduced to suppress generation of noise such us driving noise in aperture operation, exist, it is necessary to control each of exposure control values and the combination thereof according to these control conditions. However, there has been a problem that it is not necessarily the case that the corrected exposure control values satisfy these control conditions in the above-mentioned art.